Concrete is one of the commonly used materials of our time for building structures. One of the most unique properties of structural concrete is that while being capable of carrying substantial compressive loads, it is unable to carry significant tensile loads. It becomes necessary, therefore, to add reinforcing steel-bars, wires or cables to concrete to increase its tensile carrying ability in a concrete structure. The basic types of reinforcement methods in concrete structures can be separated into two groups as: conventionally reinforced structures, and pre-stressed concrete structures.
In conventionally reinforced concrete structures, deformed steel bars, called reinforcing bars, are placed in tensional stress areas of the concrete members, thus allowing the steel to carry the tensile forces and the concrete to carry compressive forces.
In pre-stressed concrete structures, the method of pre-stressing can be defined as the application of a pre-determined force or moment to a structural member, in such a manner that the combined internal stresses in the member, resulting from this force or moment and from any anticipated condition of external loading, is being confined within specific limits. Pre-stressed concrete is the result of applying this principle to concrete structural members, and eliminating or materially reducing the tensile stresses in the concrete. In this type structure, the type of reinforcement methods again separates the type of structures into two basic groups: pre-tensioned structures, and post-tensioned structures.
In pre-tensioned structures, reinforcing rods, cables or strands of high tensile strength wires (called tendons) are first pre-stretched to a certain pre-determined amount, and then high-strength concrete is placed around the reinforcing rods to form the concrete member. Once the concrete has set, pre-tensioned rods are released to introduce a compressible force into the member while the concrete holds the steel reinforcement in a tight bond, preventing slippage and sagging. The tendons used in pre-tensioned construction must be relatively small in diameter, because the bond stress between the concrete and the tendon is relied upon to transfer the stress from the tendon to the concrete. Pre-tensioning is mostly performed within individual concrete members at their manufacturing plant.
In post-tensioned structures, reinforcing rods, cables or strands of high strength wires (called tendons) are draped loosely to a profile as determined by structural analysis, and high-strength concrete is placed around them. The tendons are usually encased in a flexible plastic protective hose (called sheath or duct) to prevent the tendon from bonding to the concrete during placement and curing of the concrete. The protective sheathing remains in the structure. After the concrete has reached its pre-determined strength, the tendons are then stretched by hydraulic jacks and are securely anchored into place by some type of device mostly at their end locations. These devices are referred to as end anchorages. The end anchorages, together with the special jacking and grouting equipment used in accomplishing the post-tensioning by one of the several methods, are referred as post-tensioning systems.
In some cases, the void between the tendon and the sheath is filled with grout. In this manner, the tendon becomes bonded to the concrete section and corrosion of the steel tendon is prevented. In some other cases, the tendon is coated with grease prior to placement into a protective sheathing. Tendons of this type are not pressure grouted after stressing. This type of post-tensioning is usually referred to as an un-bonded post tensioning system. Post-tensioning of tendons is generally performed at the construction site.
In some instances, the metal components of the post-tensioning systems may become exposed to external sources of chlorides in service, such as de-icing salts, brackish water, seawater, or spray from these sources. There have been corrosion problems with either type of pre- and post-tensioning systems. However, certain aspects of corrosion of un-bonded single strand tendons are unique, and the end effects of corrosion of un-bonded single strand tendons are, in several respects, different from those of bonded conventional reinforcing or other post-tensioning systems. Thus, the methods for evaluating and repairing corrosion of single strand tendons are also different in some respects. For example, since the tendons are largely isolated from the surrounding concrete, they may not be affected by deleterious materials such as chlorides and moisture in the concrete. However, they also are not completely protected by the surrounding concrete, and can corrode if water gains access to the inside of the sheathing or anchorage and the grease protection is inadequate. Measures taken to repair and protect the surrounding concrete may not repair or reduce deterioration of the pre-stressing steel where corrosion has been initiated. The tendons usually require separate evaluation and repair.
The use of un-bonded tendons became more common during the late 1950s and early 1960s as design standards and materials standards were established. Due to their advantage over conventional type structures (shorter construction time, additional living space gain by reducing slab thickness and savings in overall cost), the use of post-tensioning gained more popularity during the late 1960s and 1970s and became one of the common type structural systems for many applications. In addition to their use in various building structures, un-bonded post-tensioning systems were used in parking structures, slab-on-grade, and nuclear power structures. The earliest incidents of corrosion of un-bonded tendons began to surface during the 1970s, since the grease used at that time period did not provide proper protection for corrosion. In the early 1980s, the Post-Tensioning Institute (PTI) recognized the structural implications of corrosion and began to implement measures to increase the durability of un-bonded post-tensioning systems. In 1985, PTI published the first performance standards for un-bonded tendons and included standards for grease (corrosion-inhibiting hydrophobic grease). The 1989 edition of American Concrete Institute (ACI 318), “Building Code Requirements for Reinforced Concrete”, made changes to include protection measures for the tendons and the quality of the concrete from the environmental conditions that would promote corrosion. Structures built prior to the adaptation of these new standards are presently experiencing corrosion problems and are in need of repair.
When a tendon has been inadvertently cut and has no tension, the tendon, along with its end anchorages, is usually replaced. The original anchors may be reused, but dislodging the old wedges is sometimes difficult and the anchors can be damaged in the process. It is usually advisable to replace the anchors with new ones since this provides the opportunity to improve the system's durability. Once free of its anchorage, strand extraction is normally not difficult. In some cases a jack can be used to pull the strand out, but this method, while reliable, is slow. Usually the loose tendon is pulled out by hand or with the assistance of a come-along or a vehicle. When a tendon is damaged, or when corrosion damage is known or believed to be localized, repairs are often made by replacement of a part of the tendon either between anchorages or on one side of the damage. If the un-damaged portions of the tendons are in good condition, the damaged section of tendon is cut away and a new piece of tendon is spliced onto the ends of the original tendon using splice couplers. The old anchors may be reused as long as the tensional force in the tendon is maintained, and if the existing wedges are not unlocked. If the tendon has been de-tensioned, then the wedges are always replaced.
In most cases, the tendon damage is localized and can be determined by investigation prior to repairs. Usually un-bonded tendons are damaged close to their end anchorages and the remaining portions of the tendons are still in good condition. In these cases, the tendons are temporarily anchored (locked-off) by installing temporary anchors at locations where the tendons are still in good condition; the damaged end of the tendons are then cut and removed. A new tendon is spliced to the end of each existing tendon and the existing anchor is replaced with a new one. In order to lock the tendons with their internal tensional force intact, the existing tendon is usually exposed through a small chipped hole, its sheathing is removed and a temporary-anchor with two sets of wedges is installed around the tendon. The tensional force at the damaged side of the tendon is then gradually released by chipping the concrete behind the existing end-anchor and by doing so, transferring the tensional force onto the new temporary-anchor. The entire operation herein is referred as “tendon lock-off”. The tendon lock-off operation is usually simple and unproblematic where the existing tendons are separate from each other, at locations where they are near to their original anchors or located in a uniform fashion. In post-tension band-lines (where post tensioning tendons are bundled to go over column lines and provide support for transfer-directional uniformly placed tendons), it is common practice to bundle several tendons together for ease of construction. Usually, four tendons per flat-bundle is the maximum recommended for floor slab construction. This limitation is pursued for two reasons. First, apart from poor consolidation, there is an increased potential of delamination at high and low profile points, and second, there is an increased probability of blow-outs at locations of horizontal curvatures due to outer strands riding over the inner ones. Unfortunately, in common practice, this recommendation is not always followed and more than four tendons are bundled together in a flat fashion. The repair of these banded tendons becomes difficult due to insufficient space between the tendons; the repair operation requires a larger exposure area for the placement of temporary-anchors, and/or most of the time, and instead of de-tensioning and repairing a single damaged tendon, it requires replacement of entire banded tendons.